Field of the Invention
An object of the present invention is an induction cooking heater whose inductor is temperature-protected.
In induction cooking ranges, each cooking heater generally has an inductor positioned beneath the glazed ceramic plate on which the containers are placed. The inductor is supplied by a generator delivering high-frequency current. The inductor is conventionally formed by one or more coils of conductor wires, made of copper for example, that get heated during operation.
Present-days coils, which are made of wires that are stranded for high-frequency applications, can withstand heating temperatures below a critical temperature, typically in the range of 200.degree. C., beyond which they undergo damage at the insulation between the wires. There is a known way, in the coil, especially at the center of the coil, of providing for a temperature sensor, for example a thermistor, to prevent the overshooting of temperature limits. However, this type of localized sensor has very localized action and does not take account of the entire surface area of the inductor. Furthermore, if the sensor does not work properly, there are situations in which the critical temperature may be reached and even exceeded. This is especially so when an empty container is placed above an inductor supplied with current, or when the-food to be cooked has to be deep-fried.
It is therefore important to provide for a system that enables the stopping of the generator or a reduction of the power delivered by this generator when the heating of an inductor is close to its critical temperature.
Furthermore, presently known induction cooking heaters generally have a magnetic circuit located beneath the inductor, the role of which is to concentrate the magnetic field generated by the inductor towards the containers to be heated so as to optimize the heating and reduce conduction disturbances to the minimum.
There are know ways of using this magnetic circuit to protect the inductor in temperature.
The document FR 2 620 894 describes, inter alia, a system for the protection in temperature of an inductor using a magnetic circuit formed by ferrite cores positioned radially on the lower side of the inductor, wherein the ferrite cores are chosen so that their Curie temperature or Curie point is slightly below the critical temperature. Thus, when the temperature of the ferrites reaches the Curie point, the ferrites suddenly lose their magnetic property and the power delivered to the cooking utensil drops swiftly thus preventing the overheating of the inductor.
A system of this kind has the following drawbacks:
The Curie point of the ferrite power materials (for example, manganese-zinc) is greater than 200.degree. C. This is already beyond the critical temperature for the inductors formed by stranded wires used in high frequency applications.
Furthermore, the fact of passing through the Curie point causes the system to stop suddenly and may furthermore result in a high level of hysteresis. Consequently, it may be necessary wait for a fairly lengthy period for the system to start working again even if the temperature of the inductor has fallen back to a permissible temperature.
The present invention is aimed at overcoming the above-mentioned drawbacks.